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All of the carbon-carbon bonds in the benzene molecule are of the same length, and it is known that a single bond is longer than a double bond. In addition, the bond length (the distance between the two bonded atoms) in benzene is greater than a double bond, but shorter than a single bond. There seems in effect to be a bond and a half between each carbon.

This is explained by electron delocalization. In order to picture this, we must consider the position of electrons in the bonds of benzene.

One representation is that the structure of the benzene molecule exists as a superposition of the forms below, rather than either form individually. This type of structure is called a resonance hybrid of the benzene molecule.

In reality, neither form really exists. Delocalisation must be explained using a higher level of theory than single and double bonds. The single bonds are formed with electrons in line between the carbon atoms - this is called Ã (sigma) symmetry. Double bonds consist of a sigma bond and another, À (pi) bond. This second bond has electrons orbiting in paths above and below the plane of the ring at each bonded carbon atom. The À-bonds are formed from atomic p-orbitals above and below the plane of ring. The following diagram shows the positions of these p-orbitals in the benzene molecule:

Being out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus there are not enough to form double bonds on all the carbon atoms, but the "extra" electrons do strengthen all of the bonds on the ring equally. The resulting molecular orbital has À symmetry.

This delocalisation of electrons is known as aromaticity, and gives benzene great stability. This is the fundamental property of aromatic chemicals which differentiates them from non aromatics.

To reflect the delocalised nature of the bonding, the benzene molecule may be depicted as a circle inside a hexagon in chemical structure diagrams.

As with most diagrams of molecular structures, the Hydrogen atoms are frequently omitted.

Benzene may result whenever carbon-rich materials undergo incomplete combustion. It is produced naturally in volcanoes and forest fires, and is also a component of cigarette smoke. Industrially, it is produced from either coal or petroleum. The steel industry 'cokes' coal. This coke is then 'cracked' to yield Benzene(63%), Toluene(14%) & Xylene(7%). Alternatively an Olefin plant will produce Benzene as a by-product of cracking naptha or gas oil.

It is one of the components of the coal tars given off when coal in converted to coke. Up until World War II, this source of benzene was sufficient to meet world demand for the chemical. However, in the 1950's, increasing demand for benzene, especially for the growing plastics necessitated the production of benzene from petroleum. Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal.

May 31st, 2014

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